This invention relates to medical ventilators for providing breaths to a patient and, more particularly, to a ventilator having a system to provide a zero flow condition during a pause phase following a mechanical inspiration.
Medical ventilator systems are used to support the respiration of patients in acute care situations and in conjunction with the administration of anesthesia to patients undergoing surgical procedures. Often, these ventilators employ an inspiratory pause period following a mechanical inspiration, which has a beneficial physiologic advantage of improving patient oxygenation under certain conditions. In addition, the pause phase allows the clinician to obtain certain physiologic information concerning that patient, specifically, it is useful to determine measures of the patient""s lung compliance and airway resistance as valuable data in evaluating the patient.
Basically, the lung compliance is an indication of the springiness or elasticity of the lungs and the airway resistance is the pneumatic resistance of the patient""s airways from the mouth where the ventilator is connected to the internal volume of the lungs. A determination of the lung compliance and resistance through use of a pause is typically accomplished through the use of the following equation derived from a simple linear model:       P    AW    =                    Q        AW            *              R        PAT              +                  V        T                    C        PAT            
Where
PAW is the Patient Airway Pressure
QAW is the Patient Airway Flow
RPAT is the Patient Airway resistance
CPAT is the Patient Lung Compliance and
VT is the delivered Tidal Volume
The values for the equation are taken during the normal inhalation of the lungs and the values are generally selected at two points, one at the end of the inspiration where the patient pressure is at a peak, PMAX and a second condition after the ventilator provides the pause so that the flow to and from the patient is zero. At that point, the patient airway pressure sample is termed PPLATEAU. Thus, by taking data at two points, there are two equations and two unknowns allowing the the lung compliance and the airway resistance of the patient to be determined as shown below.
Using the assumption that the patient airway flow (QAW) is zero at the PPLATEAU observation point, the resistance component is observed to drop out of the model equation leaving the term:       P    AW    =            P      PLAT        =                  V        T                    C        PAT            
Thus, using the PPLAT data point along with the volume delivered to the patient (VT), the equation is easily solved for the patient compliance (CPAT). With that value thus determined, the pressure (PMAX) and flow rate (QAW) obtained at the initiation of the pause period can be used to solve the model equation for the patient airway resistance. In this fashion, the clinician can readily determine measures of the patient airway resistance and the lung compliance.
In using the plateau pressure value for solving this equation however, an inaccuracy is introduced if, in fact, the flow to and/or from the patient is not truly at a zero flow condition at the end of the pause period. This could occur for a number of reasons. In some ventilator systems, fresh gas is continually being added to the patient circuit even after the end of the inspiratory cycle and thus there is a continual flow of gas to the patient during and after the ventilator pause period. Also, at the end of inspiration, the patient circuit acts as a large reservoir of gas. This gas, being at a higher pressure than the patient""s lung pressure, can continue filling the patient""s lungs during the pause period, thus again, the data obtained at the end of the pause period does not represent true zero flow conditions.
Accordingly, it would be advantageous in carrying out the foregoing calculations, to insure that the PPLATEAU is determined at zero flow conditions where it truly represents the patient""s lung pressure. In addition, ensuring that zero flow is achieved during the pause phase provides the patient with a quality of inspiratory pause that conforms with the ideal representation expected by clinicians, and may have improved clinical efficacy.
The system of the present invention therefore corrects the aforedescribed problem by providing a system for ensuring that zero flow conditions exist at the end of the pause period. Thus, the airway pressure at that point, the plateau pressure is the same as the pressure within the patient""s lungs.
In carrying out the invention, a ventilator system as described in U.S. Pat. No. 5,315,989 may be utilized and that system can determine the flow at the patient airway in order to provide an input to a controller. This controller uses the ventilator drive pressure to control the flow to a zero flow condition at the end of the pause period. In using a flow signal, a preferred algorithm continually senses that flow and readjusts the control pressure in the patient exhalation circuit until the zero flow conditions are attained. By use of the ventilator drive pressures in this manner, the operator is assured that a zero flow condition is met at the end of the pause period. Thus, accurate determinations can be made of the airway resistance and lung compliance and a more ideal pause profile is achieved.
Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiment set forth below, taken in conjunction with the accompanying drawings.